Conventionally, a NOx storage reduction catalyst has been known as a catalyst for reducing and purifying nitrogen compound (NOx) in exhaust gas discharged from an internal combustion engine. The NOx storage reduction catalyst occludes NOx in exhaust gas when the exhaust gas is in a lean atmosphere, while when the exhaust gas is in a rich atmosphere, the NOx storage catalyst makes the occluded NOx harmless by reducing and purifying the NOx with hydrocarbons contained in the exhaust gas for discharge into the atmosphere.
In addition, the NOx storage reduction catalyst also occludes sulfur oxides (hereinafter, referred to as SOx) contained in exhaust gas. When an amount of SOx occluded in the NOx storage reduction catalyst is increased, a problem is caused that the NOx purifying capacity of the NOx storage reduction catalyst is reduced. Because of this, in a case where the SOx occlusion amount reaches a predetermined amount, in order to recover the NOx storage reduction catalyst from the sulfur poisoned state by desorbing the occluded SOx from the NOx storage reduction catalyst, a so-called SOx purging operation needs to be carried out periodically in which unburned fuel is supplied to an upstream oxidation catalyst through a post injection or an exhaust pipe injection to increase the temperature of exhaust gas to a SOx desorption temperature (for example, refer to Patent Literature 1).